1. Field of the Invention
The present invention relates to an electrophotographic photoconductor used in a copying machine, a laser printer and a laser facsimile apparatus, and more particularly to an electrophotographic photoconductor comprising a charge transport layer which comprises a polymeric charge transporting material.
2. Discussion of Background
The Carlson process and other processes obtained by modifying the Carlson process are conventionally known as the electrophotographic methods, and widely utilized in the copying machine and printer. In a photoconductor for use with the electrophotographic method, an organic photoconductive material is now widely used because such a photoconductor can be manufactured at low cost by mass production, and causes no environmental pollution.
Many kinds of organic photoconductors are conventionally proposed, for example, a photoconductor employing a photoconductive resin such as polyvinyl carbazole (PVK); a photoconductor comprising a charge transport complex of polyvinyl carbazole (PVK) and 2,4,7-trinitrofluorenone (TNF); a photoconductor of a pigment dispersed type in which a phthalocyanine pigment is dispersed in a binder resin; and a function-separating photoconductor comprising a charge generating material and a charge transporting material. In particular, the function separating photoconductor has now attracted considerable attention.
When the function separating photoconductor is charged to a predetermined polarity and exposed to light, the light pass through a transparent charge transport layer, and is absorbed by a charge generating material in a charge generation layer. The charge generating material generates charge carriers by the absorption of light. The charge carriers generated in the charge generation layer are injected into the charge transport layer, and move in the charge transport layer depending on the electrical field generated by the charging process. Thus, latent electrostatic images are formed on the surface of the photoconductor by neutralizing the charge thereon. As is known, it is effective that the function separating electrophotographic photoconductor employ in combination a charge transporting material having an absorption intensity mainly in the ultraviolet region, and a charge generating material having an absorption intensity in a range from the visible region extending to the near infrared region.
Many low-molecular weight compounds have been developed to obtain the charge transporting materials. However, it is necessary that the low-molecular weight charge transporting material be dispersed and mixed with an inert polymer to prepare a coating liquid for a charge transport layer because the film-forming properties of such a low-molecular weight compound is very poor. The charge transport layer thus prepared by using the low-molecular weight compound and the inert polymer is generally so soft, that peeling of the charge transport layer easily occurs during the repeated electrophotographic operations by the Carlson process.
In addition, the charge mobility has its limit in the above-mentioned charge transport layer employing the low-molecular weight charge transporting material. The Carlson process cannot be carried out at a high speed, and the size of apparatus cannot be decreased due to the poor charge mobility in the charge transport layer when the amount of the low-molecular weight charge transporting material is 50 wt. % or less to the weight of the charge transport layer. Although the charge mobility can be improved by increasing the amount of the charge transporting material, the film-forming properties deteriorate.
To solve the problems of the low-molecular weight charge transporting material, considerable attention has been paid to a high-molecular weight charge transporting material. For example, a variety of high-molecular weight charge transporting materials are proposed as disclosed in Japanese Laid-Open Patent Applications Nos. 50-82056, 51-73888, 54-8527, 54-11737, 56-150749, 57-78402, 63-285552, 1-1728, 1-19049 and 3-50555.
However, photosensitivity of the function-separating laminated photoconductor in which a charge transport layer comprises a high-molecular weight charge transporting material is extraordinarily inferior to that of the above-mentioned laminated photoconductor employing a low-molecular weight charge transporting material in the charge transport layer.
To improve the photosensitivity of a laminated electrophotographic photoconductor in which a high-molecular weight charge transporting material is employed in the charge transport layer, it is proposed to add a low-molecular weight charge transporting material to the charge generation layer or the charge transport layer, as disclosed in Japanese Laid-Open Patent Application 5-34938. However, when the low-molecular weight charge transporting material is added to the high-molecular weight charge transporting material in the charge transport layer, the peeling of the charge transport layer easily occurs during the repeated operations. 0n the other hand, when the low-molecular weight charge transporting material is contained in the charge generation layer, the photosensitivity slightly increases, but does not attain to a satisfactory level.
As previously explained, when the charge transport layer of the function separating laminated photoconductor comprises the low-molecular weight charge transporting material and the inert polymer, the charge mobility, that is, the response speed has the limitation, and the charge transport layer easily tends to peel during the repeated operations.
The laminated photoconductor in which the high-molecular weight charge transporting material is employed in the charge transport layer can solve the above-mentioned problems, but causes a fatal problem of low photosensitivity. All the characteristics cannot be satisfied as mentioned above even though the high-molecular weight charge transporting material is used in combination with the low-molecular weight charge transporting material.